The art is replete with foam forming polymer technology. In most cases, the previously disclosed technology is aimed at the direct manufacture of open or closed cell foam form products, such as sheets, egg crates, fast food containers or the like. In most cases, the foam is produced from a suitable foam forming thermoplastic polymer, such as polystyrene, polyethylene, polypropylene, an ethylene/propylene copolymer, an ethylene/vinyl acetate copolymer or the like. Such a thermoplastic polymer is admixed with a blowing agent and, in some cases, a cross linking agent, at elevated temperature and pressure conditions sufficient to render the polymer fluid enough to be able to be thoroughly mixed with the blowing agent and the cross linking agent. This blending has in the past often been carried out in an extruder. After thorough blending this admixture in an extruder, it is suitably expelled from the extruder through a conventional dye under conditions that involve a sudden and significant drop in pressure sufficient to enable the blowing agent to expand and to form voids or closed cells within the body of the fluid polymer composition as it expands through the extruder die. As the blowing agent expands to create voids or closed cells within the extrudate, the entire composition expands and cools whereby trapping the closed cell voids within the polymer composition resulting in a significant reduction in the overall bulk density of the mixture. At the same time that the pressure is dropping, the temperature of the mixture is also reduced so as to enable the foam form material to solidify. If the pressure drop across the extruder die is sufficient, and if there is enough blowing agent in the mixture, the cells that are formed have an open cell structure; that is the cells are interconnected to each other. Thus the product so formed is generally porous. If the pressure drop is lower or if there is less blowing agent in the composition, the end product may have a closed cell structure; that is the cells are each independent and isolated from other cells by means of the cell walls. It is to be noted that while the cells are being formed, the composition is maintained under conditions that enable the cross linking agent to convert at least a portion of the thermoplastic olefin polymer into a cross linked material that has substantially stronger cell walls. The stronger cell walls tend to cause the cells to have a closed cell structure.
Generally, these manufacturing methods that have been used in the art employ at least one inflammable chemical as a blowing agent wherefore reuse of the waste portions of the cellular product composition and recapture of excess material resulting from carrying out this method has been significantly limited. Further, depending on the composition of the polymer being used, burning of the foam form product can produce toxic gases, such as cyanide, and often sends many other pollutants into the atmosphere. Other problems with existing open cell foams include poor inter-connectivity, weak structural form and uneven size of the open cells. More particularly, conventionally there is a wide variability in cell structure between the interior portion of a foam sheet product as compared to the cell structure of a surface portion of the sheet. Because of this wide variability, it is difficult to make an open cell product that has consistent absorptivity of fluids, compression set and strength, elongation to break, tensile strength, and shear strength. Further, the open cell foam form products that are known in the prior art are limited in their color penetration and in their visual performance. These adverse characteristics have limited the industrial use of these open cell foam form sheet products because it is difficult, if not impossible, to accurately and reproducibly control the size of open cells of the foam form material made according to the prior art. In particular, it is substantially impossible to specify particular characteristics such as fluid, (that is gasand liquid) retention, fluid flow or pressure drop across the thickness of the cellular products of the prior art as a function of cell size because open cell foam structures as made by prior art techniques tended to have very uneven cell sizes.
In addition, the liquid retention characteristics of the prior commercially available open cell foam materials have not found great use in filtration operations because of the inconsistency and unpredictability of the size of the open cells in the products that have been formed according to prior art techniques. In order to accomplish wide spread industrial use of foam form products with open cellular structure, such products must have a relatively constant flow of fluids (liquids or gases) throughout their structure. That is, they must have a consistent fluid pressure drop across all portions of the foam form product. An open cell structure with consistent cell geometry is required to enable these products to absorb, separate, control and/or retain fluids that come into contact with them. In order for open cell products to be used for clean up of fluid spills, the open cell structure has to enable the absorption of spilled fluids at a consistent rate and volume and retain them in a consistent manner subject to their being expelled from the foam, such as by mechanical squeezing.
In the past, open cell foam products that have been made by expelling a polymer/blowing agent composition from an extruder die have been structurally relatively weak because it has been difficult if not impossible to make the open cells substantially uniform in cell wall thickness. The distribution of blowing agent throughout the molten, pressurized polymer composition has been found to be uneven and therefore the cells produced are of uneven size and wall strength.
Reference is here made to U.S. Pat. Nos., 5,589,519, 5,859,076 6,242,503, 6,325,956, 6,958,365 and published application Ser. No. 11/314,576 that generally describe the state of the prior art related to the manufacture of open cell products. Further reference is made to expired U.S. Pat. No. 4,877,814 that shows a process of making open cell foam form sheets that is similar to the instant process. Other and additional references in the prior art will be cited as they become available.